New Zealand Journal of Geology and Geophysics abstracts
Earthquake source mechanism analysis for events between 1992 and
1997 using sparse New Zealand broadband data
Iain Matcham1
Martha K. Savage*
J. John Taber2
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
Martin Reyners
GNS Science
P.O. Box 30 368
Lower Hutt, New Zealand
1Present address: GNS
Science, P.O Box 30 368, Lower Hutt, New Zealand.
2Present
address: IRIS, 1200 New York Ave NW, Suite 800, Washington DC 20005,
USA.
*Author
for correspondence.
Abstract Data from the permanent broadband seismic
station SNZO and temporary broadband deployments have been used to test
the operation of the regional, full waveform inversion source mechanism
determination technique of Dreger and Helmberger (1991) in New Zealand.
An amplitude ratio technique similar to the AMPRAT technique, which is
widely used in New Zealand, was also tested with the same data. Tests
using synthetic data showed that the waveform inversion is not reliable
if data from only one station are available, but that the reliability
of the technique improves when a second station is added, and the
reliability is directly proportional to the azimuthal separation of the
stations. Addition of a third station further improves the results. To
test the applicability of these synthetic results, regional earthquakes
with moment tensors published between 1992 and 1997, and which were
recorded at SNZO, were investigated using the waveform inversion
technique. Where possible, additional data from temporary broadband
deployments were also included in the inversion. The results of these
tests on the 11 events support the synthetic results. While two of the
events returned source mechanisms consistent with the published
solutions, there was no known way to separate the consistent from the
inconsistent mechanisms, making it impossible to use the technique
reliably with data from only one station. When further stations were
available for addition to the inversion, the results were improved.
However, there were insufficient events recorded at multiple stations
to reach any firm conclusions. The same events were analysed using an
amplitude ratio technique plus first motion polarities from the New
Zealand short period network, and similar results were found. The
amplitude ratio technique was more reliable than the waveform inversion
when only one station (plus first motions) was used, with six of the
events returning source mechanisms consistent with the published
solutions. This was not sufficiently reliable to guarantee good
results. Addition of further stations again improved the reliability of
the results. In addition to the tests of the techniques, new source
mechanisms are presented for the 1994 June 18 Arthur’s Pass and 1995
November 24 Cass earthquakes.
Keywords moment tensor; amplitude ratio; waveform
inversion; Arthur’s Pass; Cass
G05018; Received 3 May 2005; accepted 11 November 2005; Online
publication date 28 February 2006
New Zealand Journal of Geology & Geophysics, 2006, Vol. 49:
75–89
0028–8306/06/4901–0075 © The Royal Society of New Zealand 2006
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